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Anaesthetical Monitoring of Hypnosis in Children

Cristian Tanase

“Grigore Alexandrescu” Emergency Children Hospital, Bucharest, Romania



Cristian Tanase

Intensive Care Department Grigore Alexandrescu” Emergency Children Hospital

Blv. Iancu de Hunedoara, 30-32, Sector 1, Bucharest, Romania

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The need to control the deepness of the anaesthesia becomes more and more important. The level of hypnosis given by volatile or intravenous anaesthetic agents can reach a dangerous point of too much central depression, or provoke awareness, that can be a source of unpleasant medical and legal consequences. EEG and derivates (bispectral index, entropy, spectral analysis) seem to offer feedback of the hypnotic agents’ action.

The assessment of hypnosis helps to adjust the depth of anesthesia and avoids “awareness” situations. This kind of control is so far less used in children.

EEG and derivates (bispectral index, entropy, spectral analysis) seem to offer feedback of the hypnotic agents’ action. EEG is a complex sinusoid with a frequency spectrum of 0.3 – 70 Hz. In the waking adult the medium (8-13 Hz) and fast (14-30 Hz) frequencies predominate. EEG tracing varies with age reflecting cerebral maturation processes. In newborns slow oscillations prevail, then the dominant frequency gradually increases with age, while the amplitude of the oscillations decrease. Over 1 year, EEG and its reaction caused by the general anesthetics look the same as in adults. The BIS technology includes: bispectral analysis, algorithm, bispectral index.



Entropy calculates spectral entropy of EEG. EEG is collected up from the fronto-temporal region and integrates frontal EMG. State Entropy (SE) is analyzed at a frequency range of 0.8-32 Hz and consists mostly of EEG. Response Entropy (RE) results from a 0.8-47 Hz range analysis, consisting of both EEG and frontal EMG, reflecting subcortical activity as well (nerve VII). The activation of the frontal muscle indicates inadequate anaesthesia, like nociception or impending awareness, generating a gap between RE and SE index values[1].

Spectral analysis is a mathematical process which permits the dissociation of the complex sinusoid into simpler sinusoids reflecting the constitutive oscillations (delta, theta, alpha, beta)[2].

BIS index reflects cerebral activity and not the cerebral concentration of anaesthetic. Its value diminishes during sleeping periods as well. It reflects the global metabolic changes caused by neurotrope agents. <70 = good sedation, <60 = loss of consciousness, <40 = suppression of the tracing, 0 = no activity; ideal hypnotic range = 45-60.

Connecting BIS with acting anaesthesia can produce some findings. Induction - intubation augments BIS, 50 meaning stability for tracheal intubation or laryngeal mask. Maintenance – BIS 40-60; factors that might influence BIS are medication, surgical stimulation, devices’ interference. The opiate alone does not modify BIS. Opiate + volatile agent diminish the effect of the painful stimulus on cortex activity: ideal BIS 45-60. Volatile agent without opiate imposes a BIS 25-35 for the same depth of anaesthesia, to soften the cortical response to painful stimulus. Anyway, BIS rise precedes intraanaesthetic movement or hypertension. Awakening – BIS does not perfectly correlate with movement or consciousness, but reduces the extubation time and recalls. In some emergencies, BIS helps by giving instant information, or making the differential diagnostic between residual curarisation and profound anaesthesia.

The influence of several agents on BIS is debatable. Most investigated in children is sevoflurane, including the rapid induction[3]. Particularities in infants are: for a BIS of 50, Et sevo is higher than in a 2 year old child, and BIS evolution during awakening is “on-off” vs. progressive type in 2 year old child. Some subjects experience burst-suppression with deep anaesthesia. There is a good dose-response correlation between BIS and the general anaesthetics[4].

Age (years) MAC sevoflurane
(O2 100%)
End Tidal (Et) minimal hypnotic sevoflurane
(O2 100%)
< 3 3.3 – 2.6 % ≥ 2.4 %
3 - 5 2.5 % 2 %

Epileptogenic effect of sevoflurane can consist in early agitation associated with hypertonia resolving with deepening of anaesthesia, or tonic-clonic movement occurring under deep persistent anaesthesia. Among the EEG epileptoid signs, spikes are the most frequently found. Prevention: Et sevo 1.5 MAC during maintenance and avoiding hypocapnia (table).

Awareness is as important in children as in adults. Data in children is poor, because of some particularities: heterogeneous population, MAC differences according to age, better hemodynamic tolerance, age-related EEG variability, communication difficulties. Prevention of “awareness syndrome” is done by benzodiazepines premedication, proper doses of hypnotic drugs, adding the necessary opiate to the volatile agent, avoidance of complete curarisation (when possible),BIS monitoring, informing less noise in the theatre. Incidence of awareness in children is as high as 2.7% (Isabelle Constant), 0.8% (Davidson).

BIS is relevant as an indicator of hypnosis for propofol, midazolam, isoflurane, alone or mixed with analgesic drugs. Ketamine, N2O, opiates do not depress EEG, so that they do not proportionately lower BIS. Hypothermia, hypocapnia, hypoperfusion, neuroleptic drugs reduce frequencies of EEG and alter BIS. Since opiates do not significantly modify BIS, SE, RE, these 3 do not predict the response to pain.

In the intensive care unit, EEG and derivates are useful in the control of sedation, in barbiturate coma, for the comfort of neuromuscular blocked patients, some procedures for neurologic evaluation, in detection of zones of cerebral ischemia.




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